This invention relates to a metal deposition process for microelectronic integrated-circuit device interconnections. The integrated circuits may have electrical, optical or mixed electrical and optical components. The invention also addresses the problem of providing a conductive filling or wall-coating for electrical connections between various metallic layers in integrated circuits with multiple levels of metallization. In particular it addresses the manufacture of connections between metal layers through vias, i.e. conductor-filled contact holes connecting the layers. When the via walls are near-vertical or actually vertical, the manufacturing is difficult for small and long or deep vias, such as vias of about one micrometer or less in diameter, having aspect ratios greater than 1.0. (The aspect ratio is the vertical distance between the interconnected layers divided by the diameter of the vias.)
For the manufacture of metallized vias and also of lateral inplane conductive interconnections, it is desirable that the interconnection forming process or method utilize readily obtainable materials of long shelf life. These methods and materials must form interconnecting patterns on defined areas of insulating dielectrics such as silicon dioxide, silicon nitride or glasses. In the metallization of vias, conventional processes such as evaporation (e.g. of aluminum) or sputtering (e.g. of a refractory metal like tungsten) do not provide adequate coverage of vertical or near vertical walls. The coverage of steps is also unsatisfactory. Accordingly, considerable effort has been directed at trying to devise improved methods to fill vias with conductive material. It has been recognized that these efforts, if successful, could be an important factor in improving the yield and lowering the cost of ULSI (ultra large scale integrated circuit) devices.
Tungsten is a high-melting refractory metal used in the manufacture of integrated circuit devices when a higher melting point or reduced interdiffusion with silicon are essential. Tungsten also provides a barrier against the interdiffusion of aluminum and silicon, and is usually deposited by chemical vapor deposition or by sputtering. However, neither chemical vapor deposition nor sputtering of tungsten is satisfactory for metallization of vias with steep or vertical walls, particularly when their diameters are small.
It is desirable that the method providing interconnections in VLSI or ULSI devices utilize materials and methods that are compatible with the now existing device manufacturing materials and processes. Thus, it is preferred that films containing polymeric materials be applied by spin coating. In order to avoid defects, such as pinholes in thin spun films, the spun solutions must exhibit superior wetting characteristics and the films formed upon drying must adhere well to the surfaces being coated. The solutions and the resulting films must cover difficult topographies, including walls of vertical vias. It is further desirable that the method of metallization be capable of microlithographically forming micron-size and submicron-size features consistently with existing VLSI device manufacturing processes, and that the interconnecting patterns generated act, when so desired, as sites for further selective deposition of metals such as tungsten, molybdenum, nickel or copper.
Methods to produce metal films for interconnections in devices, including methods based on the use of solutions of organic ligand-bound metal atoms and ions are known. However, the patterning of films formed of these solutions has until now been done by heating of certain film areas but not others. Such selective heating has been shown to lead to decomposition of the soluble complex, i.e. to removal of part or all of the organic ligands, so that a metallic residue was usually left in defined areas. These thermal techniques are suited for the formation of patterned films of noble metals, such as gold, platinum and palladium, because these metals, when heated with elements of the ligand, that often include oxygen, nitrogen or sulfur, in addition to carbon and hydrogen, do not form stable insulating compounds. These methods are, however, not appropriate for the deposition of tungsten and other reactive Group V and Group VI metals and are especially unsuited for their deposition from aqueous solutions. One reason for this is that Group V and Group VI elements form oxides that are stable and difficult to reduce.
Photoresist materials containing metal atoms have also been used in the fabrication of interconnecting components in VLSI devices. Their use makes it possible to reduce the number of steps in the patterning process.